Lumiphoric materials (also known as lumiphors) are commonly used with electrically activated emitters to produce a variety of emissions such as colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). Electrically activated emitters such as LEDs or lasers may be utilized to provide white light (e.g., perceived as being white or near-white), and have been investigated as potential replacements for white incandescent lamps. Such emitters may have associated filters that alter the color of the light and/or include lumiphoric materials that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit light having a second peak wavelength that differs from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphoric materials. Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) emitters, or, alternatively, by combined emissions of a blue light emitting diode (“LED”) and a lumiphor such as a yellow phosphor. In the latter case, a portion of the blue LED emissions pass through the phosphor, while another portion of the blue LED emissions is downconverted to yellow, and the blue and yellow light in combination provide light that is perceived as white. Another approach for producing white light is to stimulate phosphors or dyes of multiple colors with a violet or ultraviolet LED source.
A representative example of a white LED lamp includes a package of a blue LED chip (e.g., made of InGaN and/or GaN) combined with a lumiphoric material such as a phosphor (typically YAG:Ce) that absorbs at least a portion of the blue light (first peak wavelength) and re-emits yellow light (second peak wavelength), with the combined yellow and blue emissions providing light that is perceived as white or near-white in character. If the combined yellow and blue light is perceived as yellow or green, it can be referred to as ‘blue shifted yellow’ (“BSY”) light or ‘blue shifted green’ (“BSG”) light. Addition of red spectral output from an emitter or lumiphoric material (e.g., to yield a “BSY+R” lighting device) may be used to increase the warmth of the aggregated light output and better approximate light produced by incandescent lamps.
Many modern lighting applications require high power emitters to provide a desired level of brightness. High power emitters can draw large currents, thereby generating significant amounts of heat. Conventional binding media used to deposit lumiphoric materials such as phosphors onto emitter surfaces typically degrade and change (e.g., darken) in color with exposure to intense heat. Degradation of the medium binding a phosphor to an emitter surface shortens the life of the emitter structure. When the binding medium darkens as a result of intense heat, the change in color has the potential to alter its light transmission characteristics, thereby resulting in a non-optimal emission spectrum. Limitations associated with binding a lumiphoric material (e.g., a phosphor) to an emitter surface generally restrict the total amount of radiance that can be applied to the lumiphoric material.
In order to increase reliability and prolong useful service life of a lighting device including a lumiphoric material, the lumiphoric material may be physically separated from an electrically activated emitter (e.g., as a ‘remote lumiphor’ or ‘remote phosphor’), such as by coating a lumiphoric material on a light-transmissive carrier or other support element. LED lighting devices incorporating remote phosphors are disclosed, for example, in U.S. Pat. No. 7,234,820 to Harbers et al. and U.S. Patent Application Publication No. 2011.0215700 A1 to Tong et al.
Utilization of a remote lumiphor may also increase system efficiency and/or efficacy. An acknowledged problem with phosphor-converted white LEDs is that yellow light generated at the phosphor on top of the chip is readily absorbed back into the chip. The yellow light (generated by blue light from the LED exciting the phosphor) is omnidirectional—accordingly, just as much yellow light exits the phosphor toward the LED chip as yellow light exits away from the LED. It is estimated that between 15% and 30% of the yellow light originally generated at a phosphor layer may be reabsorbed back into a LED chip, thereby decreasing efficiency and increasing component heating. Use of remote phosphor systems permit increased efficiency. Routinely, in remote phosphor solid state lighting systems, blue LED chips are arranged in a reflective chamber (e.g., a back chamber) with a remote phosphor plate arranged at a light removal region. Because the ratio of absorbing chip area to reflective chamber area is low (typically 1:10, 1:20, or lower) and because the material used for the reflective back chamber is highly reflective (e.g., typically 95-98%) there is a much higher likelihood that yellow light emitted into the back chamber will encounter the reflector than a LED chip. Because reflective back chambers are routinely diffuse white, there is a strong likelihood that any yellow light emitted into the back chamber will make more than one “bounce” before exiting, thereby providing additional opportunities for yellow light to be absorbed into the blue chips. Thus, typical remote phosphor systems, depending on the geometric constraints, tend to provide a 5-10% improvement in system efficacy, without fully overcoming the 15% to 30% reabsorption loss associated with phosphor converted lighting devices not including remote phosphors.
This leaves between 5% and 20% of the yellow light originally emitted from the phosphor continuing to be absorbed. Dichroic filters (arranged between a LED and phosphor) have been suggested as means for allowing transmission of blue light and for reflecting yellow light (that would otherwise be emitted toward the blue LED chips) in a forward direction; however, dichroic filters have a very narrow acceptance angle for incoming light—such that light approaching a dichroic filter at a shallow angle may be reflected rather than transmitted through the filter, even when such light is of a wavelength that would otherwise be transmitted through the dichroic filter. In practice, use of a flat dichroic filter may result in light losses due to unintended blue bounces of sufficient magnitude to nullify any gain in light output attributable to improved yellow light extraction.
LED lighting devices incorporating dichroic filters and remote phosphors are disclosed, for example, in U.S. Pat. No. 7,234,820 to Harbers et al. and U.S. Patent Application Publication No. 2012/0092850 A1 to Pickard.
FIG. 1 is a schematic cross-sectional representation of a conventional lighting device 100 having a lumiphoric material (e.g., yellow phosphor) arranged in or on a lumiphor support element 140 that is spatially segregated from at least one electrically activated emitter 110 (e.g., blue LED). Traditional construction of a lumiphor support element 110 may include a glass disc that is coated with phosphor material (e.g., Calculite or Fortimo from Koninklijke Philips Electronics N.V., Netherlands). The electrically activated emitter(s) 110 are mounted on or over a substrate 101 (e.g., metal core printed circuit board (“MCPCB”) or other material for thermal management. Angled side walls 120 extending upward along an emissive surface of the emitter(s) 110 may include a highly reflective (e.g., 98-99% reflective) diffuse white material. An optical element 130 such as a dichroic filter may be placed between the emitter(s) 110 and the lumiphor element (e.g., disc) 140, with an air gap between the emitter(s) 110 and the optical element 130. The optical element 130 is intended to permit passage in a forward direction of emissions (e.g., blue light) generated by the electrically activated emitter(s) 110 and simultaneously reflect any rearward (e.g., yellow) emissions generated by lumiphoric material of the lumiphor element 140. At lateral margins of the optical element, however, a significant fraction of direct emissions generated by the emitter(s) 110 impinging on the optical element at a shallow incident angle may be reflected rearward. As shown in FIG. 1, a light beam that is substantially perpendicular to the optical element 150 is likely to result in a transmitted beam ET, whereas a light beam that impinges on the optical element 150 at a shallow angle far from perpendicular may result in a reflected beam ER that (at least initially) does not pass through the optical element 150. As a result, light extraction from the device 100 may be reduced.
The art continues to seek improved remote lumiphor lighting devices that address one or more limitations inherent to conventional devices.